1. Technical Field
The present invention is directed to stabilized Magnetoresistive (MR) and/or Giant Magnetoresistive (GMR) spin valve read elements. More specifically, the present invention is directed to apparatus and methods of making stabilized MR/GMR spin valve read elements using longitudinal ferromagnetic exchange interactions.
2. Description of Related Art
The requirement of high density magnetic storage of data on hard disk drives and magnetic tape drives has been increasing steadily for the past several years. Hard disk drives include magnetic heads for reading and writing data to the hard disk. The magnetic heads include write coils and sensors for reading data from the hard disk.
Development of magnetoresistive (MR) sensors (also referred to as heads) for disk drives in the early 1990""s allowed disk drive products to maximize storage capacity with a minimum number of components (heads and disks). Fewer components result in lower storage costs, higher reliability, and lower power requirements for the hard disk drives.
MR sensors are used for the read element of a read/write head on high-density magnetic disk and magnetic tape. MR sensors read magnetically encoded information from the magnetic medium of the disk or tape by detecting magnetic flux stored in the magnetic medium. As storage capacity of disk drives has increased, the storage bit has become smaller and its magnetic field has correspondingly become weaker. MR heads are more sensitive to weaker magnetic fields than are the inductive read coils used in earlier disk drives. Thus, there has been a move away from inductive read coils to MR sensors for use in disk drives.
During operation of the hard disk drive, sense current is passed through the MR element of the sensor causing a voltage drop. The magnitude of the voltage drop is a function of the resistance of the MR element. Resistance of the MR element varies in the presence of a magnetic field. Therefore, as the magnitude of the magnetic field flux passing through the MR element varies, the voltage across the MR element also varies. Differences in the magnitude of the magnetic flux entering the MR sensor can be detected by monitoring the voltage across the MR element.
As discussed above, MR sensors are known to be useful in reading data with a sensitivity exceeding that of inductive or other thin film sensors. However, the development of Giant Magnetoresistive (GMR) sensors (also referred to as GMR heads or Spin Valve sensors) has greatly increased the sensitivity and the ability to read densely packed data. Thus, although the storage density for MR disks is expected to eventually reach 5 gigabits per square inch of surface disk drive (Gbits/sq.in.), the storage density of GMR disks is expected to exceed 100 Gbits/sq.in.
The development of MR sensors and GMR sensors, also known as spin valve sensors, have increased the sensitivity of read heads of disk drives thereby allowing for advances in the recording density in magnetic disk recording technologies. However, MR sensors and GMR sensors suffer from domain instabilities resulting in unstable readback signals or Barkhausen noise. These domain instabilities are cause by magnetic domains that form in the soft magnetic sensing layers of MR and GMR read heads. Irregular movement of these domains induced by the signal field from the magnetic medium cause distortions in the read back voltage waveform developed across the MR/GMR sensor.
In order to solve the problems of domain instabilities, known solutions make use of end attached permanent magnets (PMs) to apply a longitudinal easy axis field to suppress the onset of Barkhausen noise in narrow track elements, i.e. MR/GMR devices whose sensing width is small. In magnetic tape medium MR/GMR read heads, xe2x80x9cnarrowxe2x80x9d is approximately less than 10 micrometers. In magnetic disk medium MR/GMR read heads, xe2x80x9cnarrowxe2x80x9d is approximately less than 2 micrometers. These permanent magnets have a number of problems, however.
First, the permanent magnets often become demagnetized or have their magnetizations altered during read head processing and read head use. For example, magnetic fields, temperature and mechanical vibration or shock can alter the magnetizations of the permanent magnets. This is often due to the fact that the most popular magnet used is Cobalt-Platinum (CoPt) or Cobalt-Platinum/Chromium (CoPtCr) magnets, which have little or no uniaxial anisotropy to help keep their magnetizations pointing in the proper direction.
Second, processing of the permanent magnet contact to the MR/GMR element, i.e. the step where the spin valve is etched into its final shape and the permanent magnets are placed on the ends of the spin valve, can cause electrical resistance problems. That is, at this step, the processing of the device forms an abutted electrical junction as well as a magnetically coupled junction. If this process contaminates this interface, the function of the permanent magnets as a stabilizing field source and as an electrical conductor connection is compromised.
Third, the field from the permanent magnets acts on the whole MR/GMR structure rather than just the sensing (free) layer. This complicates the designs of the MR/GMR sensors because the effects of the permanent magnet fields must be accounted for during the design.
Fourth, permanent magnets only really work in very narrow elements, e.g., less that 4 xcexcm. This is because the permanent magnet field penetration into the MR/GMR sensor is poor. As a result, use in wider disk head and tape head elements is ineffective and methods, such as periodic structures, have been invoked for elements in the width range of 50 xcexcm to 10 xcexcm wide. This is particularly a problem for servo read elements for tape read heads which tend to be in this width range.
Thus, it would be beneficial to have a MR/GMR spin valve read element that is stabilized to reduce domain instabilities and Barkhausen noise. It would further be beneficial to have an apparatus and method of making stabilized MR/GMR spin valve read elements that avoid the problems noted above in the prior art.
The present invention provides an apparatus and method in which a free layer of a spin valve sensor may be stabilized without using permanent magnets. In one embodiment of the present invention, the free layer is stabilized by a magnetic field applied to the free layer by a ferromagnetic layer pinned at 0 degrees. In an alternative embodiment, the magnetic field is applied by a synthetic antiferromagnetic layer.
By eliminating the necessity of having permanent magnets, the drawbacks of these permanent magnets are also eliminated. The present invention eliminates these drawbacks because, in the present invention, (a) layers used in the present invention do not rely on any retention of magnetization; (b) the effect of an effective applied field is not sensitive to adjacent magnetic structures such as shields; (c) there is no permanent magnetism to be altered or lost in the present invention; and (d) the ferromagnetic exchange interaction effect of the present invention is adjustable. These and other features and advantages will be described in, or will become apparent to those of ordinary skill in the art in view of, the following detailed description of the preferred embodiments.